Nowadays, an incredible amount of information can be obtained from the Internet and high-definition television broadcasting etc., due to the rapid advancement in optical communications technology, computers, and video equipment. In order to record and store the huge amounts of information, there has been an increase in the usefulness of terabyte-class external recording media, besides hard disks by magnetic information recording. Also now, limitations of optical memory technology by shorter wavelengths of light sources are apparent, and research has progressed to further multilayered and three-dimensional optical memories. Under such circumstances, holographic memories capable of parallel readout of image data have been researched domestically and overseas as next-generation recording media having recording capacities exceeding 1 terabyte using the three-dimensional space of the medium and also capable of high-speed readout and transfer at a data transfer speed of 10 gigabits per second.
A holographic memory records an interference pattern formed by a signal light (or object light) and a reference light as a spatial distribution of the refractive index. Here, a holographic memory recording device encodes data two-dimensionally, imparts the coded data to a signal light by a spatial light modulator, and records the same on a recording medium by page. The light modulator has a large number of pixels arrayed in rows and columns, and the individual pixels can independently transmit and block light to two-dimensionally express an arbitrary ON/OFF bit pattern.
A signal light, when being transmitted through the light modulator, is spatially modulated in light intensity according to a two-dimensional pattern of the spatial light modulator. Shifting the incident angle and incident position of the signal light on a holographic memory recording medium enables multiplex recording in a manner superimposed on the same part of the medium, whereby performing large-capacity data recording.
As above, the recording density of a holographic memory is given by the data volume per page and the number of multiplex recordings per unit area. It is essential to increase the number of multiplex recordings in order to realize a recording capacity of 1 TB in a 5-inch disc size. Here, for increasing the number of multiplex recordings, it is necessary to suppress the bit error rate while reducing the recording interval in the thickness direction of a recording layer of the disc. Block coding is used as a method for suppressing the bit error rate.
The block coding is a method of dividing one page into small areas, in which reconstruction of a signal of 0 or 1 is performed. In a holographic memory, 3:16 coding and the like is often used in which only three pixels are lit up in an area of 4×4 (pixels) (refer to, for example, Patent Document 1).
For example, in 2:4 coding in which 2 pixels are lit up among 2×2 (pixels), the number of combinations that can be taken as code patterns is 4C2=6 patterns. Because 22bits=4 and 23bits=8, a code of 2×2=4 pixels (that is, 4 bits where 1 pixel is 1 bit) is constructed in response to 2 bits of signal data based on 4<6<8. In this case, the coding rate is 2/4=0.5.
Also, for example, in 2:9 coding in which 2 pixels are lit up among 3×3 (pixels), the number of combinations that can be taken as code patterns is 9C2=36 patterns. Because 25bits=32 and 26bits=64, a code of 3×3=9 pixels (that is, 9 bits) is constructed in response to 5 bits of signal data based on 32<36<64. In this case, the coding rate is 5/9=0.5556.
Also, for example, in coding in which 3 pixels are lit up among 4×4 (pixels), the number of combinations that can be taken as code patterns is 16C3=560 patterns. Because 29bits=512 and 210bits=1024, a code of 4×4=16 pixels (that is, 16 bits) is constructed in response to 9 bits of signal data based on 512<560<1024. In this case, the coding rate is 9/16=0.5625.
That is, in block coding that has been used in holographic memories, the information volume per page becomes approximately ½. As described above, because the recording density of a holographic memory is given by the multiplication of the data volume per page and the number of multiplex recordings per unit area, it has been necessary, in order to increase the recording capacity, to increase the information volume per page and reduce the bit error rate so as to increase a larger multiplexing number.    [Patent Document 1] JPA 2001-75463